1. Field of Invention
This invention relates to novel and improved memory devices and in particular to frequency memory apparatus in which an input signal is stored in a regenerative ocsillatory loop.
Frequency memories are useful in repeater aplications wherein it is necessary to regenerate an input signal with substantially no deviation in frequency. For example, frequency memories are especially useful in jamming applications wherein it is desirable to regenerate a signal in response to a received signal such that the frequency of the regenerated signal closely approximates that of the received signal.
2. Prior Art
Prior art frequency memories generally involve the recirculation of a signal in an oscillatory loop which includes an amplifier and a delay line. In order to sustain the oscillation, the total gain around the loop must be substantially unity and the net phase delay or shift in the loop must be an integral multiple of 2.pi. radians (360.degree.) at the frequency of the signal to be stored. For a phase delay around the loop of T (which is due largely to the delay line), oscillation can be sustained within the loop at any of a set of frequencies with frequency spacing .DELTA.F, where .DELTA.F=1/T. At any of these frequencies (sometimes called loop modal frequencies) and unity gain, ideal frequency memory occurs.
In such prior art frequency memories, the phase delay T does not often equal a multiple of 2.pi. radians at the input signal frequency. This results in spectral spreading of the output power. That is, the output power is not concentrated at the input frequency. When the phae delay T is an integral multiple of 2.pi. radians at the input signal frequency (in phase condition), the output power attenuation is minimized. However, as the phase delay or shift deviates from the in phase condition, the output power becomes more and more attenuated with maximum attenuation occurring at the 180.degree. phase shift condition.